1. Field of the Invention
The present invention relates to a photonic crystal device with a variable photonic crystal structure.
2. Description of the Related Art
Various types of photonic crystals, having a one-, two- or three-dimensional lattice, have been reported. A photonic crystal having the simplest structure is formed by alternately stacking two types of dielectric thin films with mutually different dielectric constants one upon the other.
The structure of the one-dimensional photonic crystal disclosed in John D. Joannopoulos, Robert D. Meade and Joshua N. Winn, “Photonic Crystals: Molding the Flow of Light”, translated by Hisataka Fujii and Mitsuteru Inoue, 1st printing of 1st edition, published by Corona Publishing Co., Ltd. on Oct. 23, 2000 (ISBN 4-339-00727-7), p. 42, FIG. 4-1 will be described with reference to FIG. 28. The one-dimensional photonic crystal 1201 shown in FIG. 28 includes low-dielectric-constant layers 1202 and high-dielectric-constant layers 1203 that are stacked alternately. The low-dielectric-constant layers 1202 and high-dielectric-constant layers 1203 are made of dielectric materials that transmit an electromagnetic wave 1204.
In the example illustrated in FIG. 28, the unit cell (with a lattice constant a) of the photonic crystal is formed by a pair of low- and high-dielectric-constant layers 1202 and 1203. A number of such unit cells are arranged in the z-axis direction, thereby defining a one-dimensional periodic structure.
Hereinafter, it will be described how the one-dimensional photonic crystal 1201 works.
If the electromagnetic wave 1204 that has propagated in the z-axis direction is incident perpendicularly onto the lower surface of the one-dimensional photonic crystal 1201, the electromagnetic wave 1204 may be unable to transmit through the one-dimensional photonic crystal 1201 depending on its frequency. Such a frequency range in which the electromagnetic wave 1204 is forbidden to transmit (i.e., a forbidden frequency band) is called a “photonic band gap (PBG)”. The PBG has a similar property to that of the electron's band gap of a normal crystal, and depends on the lattice structure of the photonic crystal. In the one-dimensional photonic crystal 1201, the PBG frequency band changes with the dielectric constants of the low- and high-dielectric constant layers 1202 and 1203 and the magnitude of the lattice constant a.
The PBG is present for the following reason.
In the one-dimensional photonic crystal 1201, the incoming electromagnetic wave 1204 is partially reflected from every interface between the low- and high-dielectric-constant layers 1202 and 1203, thereby producing a reflected wave. There are a lot of interfaces in the one-dimensional photonic crystal 1201, thus producing a number of reflected waves. If the wavelength of the electromagnetic wave 1204 matches the lattice constant a and if the reflected waves are in phase with each other and superposed one upon the other, then those reflected waves will interfere with each other and intensify each other without attenuating. In that case, if there are a good number of unit cells in the propagation direction of the electromagnetic wave 1204, then the incoming electromagnetic wave 1204 will be reflected substantially totally. More specifically, when a phase difference between a wave reflected from an interface and a wave reflected from another interface that is adjacent to the former interface is an integral multiple of ±2π, all of those electromagnetic waves 1204 reflected from the respective interfaces will intensify each other. As a result, an intense reflected wave will be produced by the photonic crystal 1201 as a whole.
If a sufficiently large number of unit cells are arranged, then the photonic crystal 1201 will produce zero transmitted waves because it is a passive circuit and due to the energy conservation law. Consequently, the PBG is produced.
This feature of the photonic crystal is used in not just the field of optics but also various other fields of application. In the field of radio frequency communications, for example, this feature is taken advantage of to improve the radiation characteristic of an antenna and to reduce crosstalk between transmission lines.
It was proposed that the characteristic of a microstrip antenna, including a conductor pattern on a dielectric substrate, be improved by using the photonic crystal. A conventional microstrip antenna has considerable directivity for electric fields that are parallel to its dielectric substrate and for E-plane (which is defined for a linearly polarized antenna as a plane containing the electric field vector and direction of maximum radiation). Accordingly, electromagnetic waves radiated from the microstrip antenna with such directivity are easily coupled to surface wave modes having the capability of propagating on the dielectric substrate. Thus, unwanted leakage of electrical power, not contributing to radiation, is likely to occur to produce diffracted waves at the edges of the dielectric substrate. As a result, the directivity of the antenna is disturbed, which is a problem.
To overcome such a problem, it is effective to arrange the photonic crystals around the antenna. If the PBG is matched with the operating frequency of the antenna, then no electromagnetic waves could propagate parallel to the surface of the dielectric substrate. As a result, such leakage of electrical power, not contributing to radiation, can be reduced significantly.
However, the conventional photonic crystal cannot change its lattice constant a dynamically, i.e., cannot change the frequency of appearance of the PBG as required.
In order to overcome the problems described above, a primary object of the present invention is to provide a photonic crystal device that can easily change the frequency range in which the PBG appears.